The present invention relates generally to absorbent garments. Particularly, it relates to thin, folded, xe2x80x9cpulplessxe2x80x9d absorbent cores for disposable absorbent garments having improved core integrity in-use, high SAP efficiency, and surge capacity.
Traditionally, disposable absorbent garments, such as infant diapers or training pants, adult incontinence products and other such products, are constructed with a moisture-impervious outer backing sheet (or xe2x80x9cbacksheetxe2x80x9d), a moisture-pervious body-contacting inner liner sheet (or xe2x80x9ctopsheetxe2x80x9d), and a moisture-absorbent core (or xe2x80x9cabsorbent corexe2x80x9d) sandwiched between the liner sheet and the backing sheets.
Much effort has been expended to find cost-effective materials for absorbent cores which display good liquid absorbency and retention. Superabsorbent polymers or superabsorbent materials in the form of granules, beads, fibers, bits of film, globules, etc., have been favored for such purposes. Such superabsorbent materials are generally polymeric gelling materials which are capable of absorbing and retaining even under moderate pressure large quantities of liquid, such as urine and body wastes, relative to their weight. The term xe2x80x9csuperabsorbent polymerxe2x80x9d is often abbreviated as xe2x80x9cSAP.xe2x80x9d
The superabsorbent material is generally a water-insoluble but water-swellable polymeric substance capable of absorbing water in an amount which is at least ten times the weight of the substance in its dry form. In one type of superabsorbent material, the particles or fibers may be described chemically as a crosslinked, sodium-neutralized polyacrylate. Included in this class of materials are such modified polymers as sodium-neutralized crosslinked polyacrylates and polysaccharides including, for example, cellulose and starch and regenerated cellulose which are modified to be carboxylated, phosphonoalkylated, sulphoxylated or phosphorylated, causing the SAP to be highly hydrophilic. Such modified polymers may also be crosslinked to reduce their water solubility.
The ability of a superabsorbent material to absorb liquid is dependent upon the form, position and/or manner in which particles of the superabsorbent material are incorporated into the absorbent core. Whenever the superabsorbent material in the absorbent core is wetted, it swells and forms a gel. Gel formation can block liquid transmission into the interior of the absorbent core, a phenomenon called xe2x80x9cgel blockingxe2x80x9d. Gel blocking prevents liquid from rapidly flowing or wicking past the xe2x80x9cblockingxe2x80x9d particles of superabsorbent, causing portions of a partially hydrated core to become inaccessible to multiple doses of liquids, such as urine, water and saline solutions. Further absorption of liquid by the absorbent core must then take place via a diffusion process within the polymer gel. This is typically much slower than the rate at which liquid is applied to the core. Gel blocking often leads to leakage from the absorbent article well before all of the absorbent material in the core is fully saturated.
Despite the incidence of gel blocking, superabsorbent materials are commonly incorporated into absorbent cores because they absorb and retain large quantities of liquid, even under load. However, in order for superabsorbent materials to function, the liquid being absorbed in the absorbent structure must be transported to unsaturated superabsorbent material. In other words, the superabsorbent material must be placed in a position to be contacted by liquid. Furthermore, as the superabsorbent material absorbs the liquid, it must be allowed to swell so as to maintain a capillary structure within the absorbent core to distribute liquid.
Adequate absorbency of liquid by the absorbent core at the point of initial liquid contact and rapid distribution of liquid away from this point is necessary to ensure that the absorbent core has sufficient capacity to absorb subsequently deposited liquids. Prior art absorbent cores have thus attempted to absorb quickly and distribute large quantities of liquids throughout the absorbent core while minimizing gel blocking during absorption of multiple doses of liquid.
In general, some of the most important performance attributes of an absorbent core of a diaper (or any other absorbent garment) are functional capacity, rate of absorption, and core stability in use. Absorption under load or AUL of the core is a good measure of functional capacity and the rate at which that absorption occurs. Core AUL is a function of both SAP basis weight (mass of SAP per unit area), physical properties or AUL of the SAP, and absorbency of other materials used in the core. Baby diaper cores that contain only fluff pulp and a high gel strength SAP maintain adequate functional absorbency and SAP efficiency if the core contains less than about 50 percent SAP. Fluff/SAP diaper cores containing more than 50 percent SAP by weight result in lower functional absorbency because of gel blocking. Although fluff/SAP cores at greater than 50 percent SAP can provide adequate absorbency, the overall basis weight of the core must be increased to compensate for the lower efficiency of the SAP. Increasing the basis weight decreases the performance/cost ratio of the absorbent core, making them uneconomical. Also, increased basis weights tend to affect the fit and comfort of the garment, as well as impacting unfavorably packaging and shipping costs.
The comfort, fit and wearability of a diaper is greatly improved by reducing the thickness of the absorbent core. However, absorbent articles having thin absorbent cores are generally much less effective than absorbent articles having thick absorbent cores. Thin, xe2x80x9cpulplessxe2x80x9d absorbent cores generally suffer from poor core stability in-use, poor SAP efficiency due to gel blocking and low absorption rate (or surge capacity).
Therefore, it is highly desirable to provide an absorbent article with a thin, pulpless, absorbent core having improved core stability in-use, SAP efficiency and absorption rate. Garments made from such thin absorbent cores would also exhibit improved comfort, fit and wearability without sacrificing the liquid absorption characteristics of the absorbent article.
The present invention as defined by the preferred embodiments is designed to overcome the foregoing and other deficiencies of prior art absorbent garments while providing a thin absorbent core made from a folded absorbent laminate.
It is therefore an object of the invention to provide a thin absorbent garment having an improved ability to retain fluids.
It is another object of the invention to provide an absorbent garment having improved comfort, fit and wearability.
It is yet another object of the invention to provide an absorbent garment having a thin absorbent core.
It is a further object of the invention to provide an absorbent garment with a pulpless absorbent core having SAP as a substantial percentage of its basis weight, generally greater than 50%, the absorbent core being substantially free of gel blocking, i.e., the core retaining high SAP efficiency.
It is still yet a further object of the invention to provide an absorbent garment with an absorbent core having high dry and wet strength for processing and in-use performance.
It is still yet a further object of the invention to provide a thin, high density absorbent laminate or composite having good liquid absorption characteristics and especially surge capacity or absorption rate.
It is yet a further object of the invention to provide an absorbent core comprising one or more laminates wherein one of the layers of the laminate or laminates is a high-density, high SAP-containing layer.
These and other objects of the invention are achieved by a disposable absorbent article comprising a thin, folded, pulpless absorbent core. Preferably, the absorbent core includes a laminate structure (or an xe2x80x9cabsorbent laminatexe2x80x9d or xe2x80x9claminatexe2x80x9d or a xe2x80x9ccompositexe2x80x9d) comprising an absorbent layer sandwiched between an upper tissue layer and a lower tissue layer. In one embodiment, the upper and lower layers are made from materials selected from the groups consisting of tissue, airlaid fluff pulp and synthetic nonwoven. In various embodiments, a tissue may have a basis weight of about 10-40 g/sm, an airlaid fluff pulp may have a basis weight of about 35-100 g/sm, and a synthetic nonwoven may have a basis weight of about 15-25 g/sm. Preferably, the upper layer has high liquid porosity and the lower layer has a high wet strength. In various embodiments, the upper layer may have a porosity greater than about 40 ml/cm2/min, and the lower layer may have a machine directional wet tensile strength greater than about 200 gm/inch.
The absorbent layer comprises SAP mixed with fibrous or particulate additives or materials or a mixture of the fibrous and particulate additives that are effective for maintaining high SAP efficiency. These additives or materials will hereinafter be referred to as xe2x80x9cSAP stabilization additives or materials,xe2x80x9d xe2x80x9cstabilization additives or materialsxe2x80x9d or xe2x80x9cSAP efficiency additives or materials.xe2x80x9d They generally allow for an efficient utilization of the SAP material at high SAP concentrations. Typically, prior art absorbent cores at SAP concentrations greater than about 50 percent by weight exhibited SAP efficiencies of less than about 70 percent. Pulpless absorbent cores according to preferred embodiments of the invention, having SAP concentrations from about 50 to about 95 percent by weight, exhibit SAP efficiencies of at least about 70 percent, preferably greater than about 80 percent and more preferably greater than about 90 percent.
The SAP may in one embodiment have an FVAUL greater than about 15 cm3/60 ml, and may have a particle size distribution comprised of particles having particle diameters up to about 600 pm. Preferably, the SAP material is dispersed substantially homogeneously within a continuous matrix of the fibrous or particulate non-SAP additives (or their mixture), which may comprise about 5 to about 50 percent by weight of the absorbent layer. In various embodiments, the absorbent layer may comprise up to about 20 percent by weight fluff wood pulp fibers or up to about 5 percent by weight thermally bondable fibers. More preferably, the SAP material should form a substantially continuous phase and the fibrous or particulate material (or their mixture) is dispersed within the voids formed between the SAP material.
In one embodiment the absorbent layer has a basis weight of about 100 to 400 g/sm. In another embodiment, the upper layer comprises a tissue layer having a basis weight of about 16 g/sm, the lower layer comprises a tissue layer having a basis weight of about 22 g/sm, and the absorbent layer has a fiber basis weight of about 40 g/sm and a SAP basis weight of about 160 g/sm.
The SAP stabilization additives in combination with a careful selection of adhesives and application patterns of the adhesives minimize gel blocking. Moreover, the adhesives and adhesive patterns employed in the preferred embodiments of this invention selectively modify the porosity of the upper and lower tissue layers (of the laminate) and improve the overall absorbent core integrity in use. A hydrophilic hot melt adhesive having an air/water advancing contact angle of less than about 30xc2x0 may be used in various embodiments of the invention.
In one embodiment, the absorbent layer comprises a layer of fibers, and a portion of the layer of fibers is bound to both the upper and lower layers. In this embodiment, a portion of the SAP material is adhesively bound to the layer layer while another portion of said SAP material is loosely contained within the fibers of the absorbent layer. The layer of fibers may have a basis weight of about 40 g/sm while the SAP has a basis weight of about 160 g/sm. In addition, about 120 g/sm of SAP may be attached to the lower layer and about 40 g/sm of SAP may be loosely contained within the fibers.
In still another embodiment, the SAP is bound to a central portion of the lower layer by applying an adhesive at a relatively high coverage along the central portion of the lower layer. The adhesive decreases the porosity of the central portion of the lower layer. Additional SAP is bound to outer portions of the lower layer by applying an adhesive at an intermediate coverage to maintain porosity in the outer portions of the lower layer. The layer of fibers is bound to the upper layer by applying an adhesive at a relatively lower coverage to maintain the highly liquid porous structure of the upper layer. In this embodiment, the lower layer has a basis weight of about 22 g/sm, and the central portion of the lower layer is about 125 mm. The adhesive is applied to the central portion at about 2 to 8 g/sm to achieve a liquid porosity for the central portion of the lower layer of less than about 10 ml/cm2/min. The outer portions of the lower layer are about 38 mm and the adhesive is applied to the outer portions of the lower layer at about 2-5 g/sm to achieve a liquid porosity of greater than about 15 ml/cm2/min. In addition, the layer of fibers is attached to the upper layer by applying an adhesive at a basis weight of less than about 2 g/sm to maintain a liquid porosity of the upper layer of greater than about 40 ml/cm2/min.
The fibrous or particulate additives are used at an effective amount for maintaining high SAP efficiency even at high SAP concentrations. Fibrous additives preferably include cellulose acetate fibers, rayon fibers, Courtauld""s LYOCELL fibers, polyacrylonitrile fibers, polypropylene and polyester fibers, surface-modified (hydrophilic) polyester fibers, surface-modified polyolefin/polyester bicomponent fibers, surface-modified polyester/polyester bicomponent fibers, cotton fibers, cotton linters or blends thereof. The fibrous additives also may be a tow of cellulose acetate or polypropylene fibers or polypropylene fibers formed by meltblown or DYNAFIBER UFD polymer spray nozzles. The particulate additives may comprise insoluble, hydrophilic polymers with particle diameters of 100 um or less. In various embodiments, the particulate additives comprise.insoluble, dried beet fiber or other vegetable or fruit by-products. Preferred particulate additives include potato, corn, wheat, and rice starches.
The absorbent core according to the preferred embodiments of the present invention includes an absorbent laminate having an average density of from about 0.10 to about 0.40 grams per cubic centimeter, preferably from about 0.15 to about 0.35 grams per cubic centimeter, and most preferably about 0.20 to about 0.30 grams per cubic centimeters. Also the thickness of preferred absorbent laminate is less than about 0.5-1.1 millimeters, preferably from about 0.6 to about 1.0 millimeters, and more preferably from about 0.75 to about 0.85 millimeters.
Finally, the folded structure of the absorbent core exhibits substantially improved liquid absorption rate or surge capacity as compared to conventional pulp/SAP cores containing about 35-50% SAP. Preferably, the absorbent core, which may be a roll good, is C-folded folded so as to form a longitudinally extending central channel between side folded areas and having a front edge and a back edge. In one embodiment, the C-folded core has a width of about 110-130 mm, the side folded areas have a width of about 20-40 mm, and the central channel has a width of about 50-70 mm. It is postulated without wishing to limit the invention in any way that the improved absorption rate of the folded structure is generally associated with the increased free volume and surface area of the folded core.
In addition to the foregoing advantages, absorbent garments having absorbent cores according to the present invention exhibit improved comfort, fit and wearability of the garment. Such absorbent garments or articles include a liquid permeable topsheet, a liquid impermeable backsheet and the folded absorbent core of this invention. A transfer layer may be positioned between the topsheet and the upper layer of the absorbent laminate of the absorbent core, and adhesively secured to the upper layer and to the topsheet. Such garments also may have a pair of inner waste flaps positioned on the topsheet to contain and direct the flow of urine to the central channel of the absorbent core. Furthermore, such garments also may have a full- or partial-length internal spreading layer, which may be a high bulk tissue, an airlaid nonwoven, or a synthetic high loft nonwoven. Such a high bulk tissue layer may have a basis weight of about 15-40 g/sm, such an airlaid nonwoven may have a basis weight of about 35-100 g/sm and such a synthetic high loft nonwoven may have a basis weight of about 20-80 g/sm. Further, due to the relatively low thickness of the resulting product, less packaging material is needed for the same amount of product, which in turn results in lower shipping and handling costs